1. Field of the Invention
The present invention relates to a structure capable of selectively emitting multiple-wavelength light using a photonic crystal.
2. Related Background Art
Up to now, several methods of selectively emitting light having a predetermined wavelength have been known. Such a method is broadly classified into an active light emitting method and a passive light emitting method. With respect to the active light emitting method, there are, for example, a method using a light emitting diode and a method using plasma light emission. Light is emitted with a specific wavelength determined according to a medium to be used. A method of applying excitation energy to a fluorescent material to obtain light having a desirable wavelength has been also known. A method using a wavelength selective filter capable of transmitting only light having a specific wavelength, of light having a relatively wide wavelength region, such as white light has been known as the passive light emitting method.
In any of the above-mentioned methods, light having a predetermined wavelength distribution is emitted from a light emitting structure. Therefore, for example, when a display apparatus is to be produced using such a conventional light emitting structure, a plurality of conventional light emitting structures for emitting light having different wavelengths are arranged to compose a group, thereby expressing the entire group by an arbitrary color. Thus, for example, when a display apparatus is composed of a combination of pixels, each of which has a structure for emitting light in color of R, G, or B, it is necessary to alternately arrange the pixels, so that the resolution of an image which can be displayed thereon corresponds to ⅓ of the number of actual pixels.
Japanese Patent Application Laid-Open No. H06-283812 discloses a structure for extracting multiple-wavelength laser light from a single light emitting device. More specifically, there is disclosed a structure in which a plurality of semiconductor lasers each having a multi-layer film resonator structure is stacked to extract multiple-wavelength laser light.
On the other hand, it has been known that a photonic crystal is used for a light-emitting device. A photonic band gap (PBG) that allows almost no transmission of light having a predetermined wavelength region is used to confine light in a point defect provided in the photonic crystal, with the result that light energy can be concentrated to emit light with high efficiency (U.S. Pat. No. 5,784,400 B). When a light emitting device having a high light confining effect and high light emitting efficiency is to be realized, it is particularly effective to use a three-dimensional photonic crystal having a PBG in all directions. Such a light emitting device can be applied to various applications such as optical communications and display apparatuses, so that a structure having a wide operating wavelength band is required therefore because of its wide application range. For example, when the display apparatus is constructed, a light-emitting device for generating light having wavelengths corresponding to the wavelengths of R, G, and B which are three primary colors of light is required.
Assume that the structure in which the plurality of semiconductor lasers each having the multi-layer film resonator structure are stacked as disclosed in Japanese Patent Application Laid-Open No. H06-283812 is used to selectively extract multiple-wavelength light from the single light-emitting device. In this case, reflectance of a reflection multi-layer film is insufficient to light having a specific wavelength. Therefore, in each unit light-emitting device, high light emitting efficiency is not obtained and a heating value is likely to increase. In addition, because the reflection multi-layer film of each unit light-emitting device has predetermined reflectance to light emitted from other light emitting devices, light emission is mutually inhibited. Therefore, light-emitting efficiency becomes lower and a heating value is likely to increase. As a result, for example, when a plurality of light emitting devices, each of which can emit light having a plurality of wavelength regions are integrated to form a display apparatus, it is hard to sufficiently improve an integration density because of an increase in heating value.
In the conventional technique, the resonator structure using the reflection multi-layer film is a one-dimensional thin film structure. Therefore, it is hard to perform mode pattern control of light confined in a resonator in-plane direction.
In contrast to this, when the photonic crystal is used for the light emitting structure, it is hard to control a wavelength region of a complete photonic band gap which can be realized by the three-dimensional photonic crystal. For example, in the case of a photonic crystal having an inverse diamond opal structure (refraction index of high-refraction index material: 2.33, refraction index of low-refraction index material: 1.00, and PBG central wavelength: 550 nm), the complete photonic band gap can be obtained in only a band of about 50 nm. Therefore, it is hard to control all light corresponding to the wavelengths of R, G, and B using the single three-dimensional photonic crystal.
It has been known that the PBG widens as a difference of refraction index between a high-refraction index material and a low-refraction index material that composes the photonic crystal becomes larger. However, a material that is transparent in a visible wavelength region generally has a low refraction index, so that it is hard to obtain a wide PBG (material: refraction index, TiO2: 2.3, Ta2O5: 2.1, CeO2: 2.05, ZrO2: 2.03, GaN: 2.4, LiNbO3: 2.2, LiTaO3: 2.1, and BaTiO3: 2.3). The above-mentioned materials each have a lower refraction index than that of each of materials generally used in an infrared wavelength region (material: refraction index, Si: 3.4, GaAs: 3.6, and Ge: 4.0). Therefore, it is hard to realize wide band operation in the case where a use wavelength region is particularly the visible wavelength region.